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Author name code: zwaan
ADS astronomy entries on 2022-09-14
author:"Zwaan, Cornelis" OR author:"Zwaan, Kees"
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Title: Solar and Stellar Magnetic Activity
Authors: Schrijver, C. J.; Zwaan, C.
2008ssma.book.....S Altcode:
1. Introduction: solar features and terminology; 2. Stellar structure;
3. Solar rotation and meridional flow; 4. Solar magnetic structure;
5. Solar magnetic configurations; 6. Global properties of the solar
magnetic field; 7. The solar dynamo; 8. The solar outer atmosphere;
9. Stellar outer atmospheres; 10. Mechanisms of outer-atmospheric
heating; 11. Activity and stellar properties; 12. Stellar magnetic
phenomena; 13. Activity and rotation on evolutionary time scales;
14. Activity in binary stars; 15. Propositions on stellar dynamos;
Appendix I: unit conversions; Index.
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Title: VizieR Online Data Catalog: Ca II H and K Measurements Made
at MWO (Duncan+ 1991)
Authors: Duncan, D. K.; Vaughan, A. H.; Wilson, O. C.; Preston,
G. W.; Frazer, J.; Lanning, H. H.; Misch, A.; Mueller, J.; Soyumer,
D.; Woodard, L.; Baliunas, S. L.; Noyes, R. W.; Hartmann, L. W.;
Porter, A.; Zwaan, K.; Middelkoop, F.; Rutter, R.; Mihalas, D.
2005yCat.3159....0D Altcode:
Summaries are presented of the photoelectric measurements of stellar
CaII H and K line intensity made at Mount Wilson Observatory during
the years 1966-1983. These results are derived from 65,263 individual
observations of 1296 stars. For each star, for each observing season,
the maximum, minimum, mean, and variation of the instrumental H and
K index "S" are given, as well as a measurement of the accuracy of
observation. A total of 3110 seasonal summaries are reported. <P />These
observations were obtained with two instruments, HKP-1 and HKP-2. The
HKP-2 instrument is a four-channel chopping spectrometer which records
counts in 1.09{AA} FWHM triangular bandpasses centered in the H and
K lines as well as in two 20{AA} reference bandpasses centered on
3901.067 and 4001.067{AA}. The stellar activity is expressed by the
index S defined as <P />S = {alpha} (Nh+Nk)/(Nr+Nv) <P />where Nh and
Nk are the counts (corrected from background) in the H and K lines,
Nr and Nv those in the reference continuum bandpasses, and {alpha}
is a constant of proportionality used to correct for night-to-night
instrumental variations. Higher values of S generally correspond to
higher levels of chromospehric activities. <P />Factors which effect the
ability to detect stellar activity variations and accurately measure
their amplitudes such as the accuracy of the H and K measurements and
scattered light contamination are discussed. Relations are given which
facilitate intercomparison of "S" values with residual intensities
from ordinary spectrophotometry, and for converting measurements to
absolute fluxes. <P />(1 data file).
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Title: Origin and Evolution of Filament-Prominence Systems
Authors: Martens, Petrus C.; Zwaan, Cornelis
2001ApJ...558..872M Altcode:
We present a “head-to-tail” linkage model for the formation,
evolution, and eruption of solar filaments. The magnetic field
structure of our model is based on the observation that filaments
form exclusively in filament channels with no apparent magnetic
connections above the polarity inversion line. The formation of
a filament in this configuration is driven by flux convergence
and cancellation, which produces looplike filament segments with
a half-turn. Filament segments of like chirality may connect and
form long quiescent filaments. Such filaments are stabilized through
footpoint anchoring until further cancellation at the footpoints causes
their eruption. The eruption restores the original filament channel so
that filament formation may resume immediately. We then demonstrate
that the combined workings of Hale's polarity law, Joy's law, and
differential rotation introduce a strong hemispheric preference
in the chirality of filaments formed poleward of the sunspot belt,
which is in agreement with observations. We analyze the magnetic fine
structure of filaments formed through our model and find consistency
with the observed hemispheric preference for barb orientation and
a simple explanation for barb formation. Finally, we consider the
flux tubes retracted below the surface in the process of filament
formation. We show that every cancellation event that generates a
filament obeying the hemispheric chirality preference injects a flux
tube below the surface with a poloidal field opposite that of the
ongoing cycle. We suggest that this pattern of submergence of flux
represents the specific mechanism for the reversal of the poloidal
flux in a Babcock-Leighton-Durney-type model for the solar dynamo.
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Title: Solar and stellar magnetic activity
Authors: Schrijver, Carollus J.; Zwaan, Cornelis; Priest, Eric R.
2001PhT....54i..54S Altcode:
No abstract at ADS
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Title: Sources of the Slowly-Varying Component of Solar Microwave
Emission and their Relationship with their Host Active Regions
Authors: Tapping, K. F.; Zwaan, C.
2001SoPh..199..317T Altcode:
Daily surveys of the solar disc made at 2.8 cm wavelength over
the period 1-13 November 1981, complemented by magnetograms and
Hα filtergrams, are used to examine the relationship between
sources of the slowly varying component of solar radio emission and
properties of their host regions. Two classes of source are noted:
diffuse and compact. Sources are designated compact when smaller than
40”. The diffuse sources may be explained in terms of free - free
thermal emission from trapped plasma in loops overlying the active
region. The great majority of compact sources can be accounted for in
terms of gyroresonance from thermal electrons in the strong magnetic
fields overlying sunspots. A small minority are less amenable to
this explanation. They are associated with magnetic complexity and
dynamism, lie close to magnetic polarity reversals, and could be
non-thermal. Microwave sources are an evolutionary feature common to
all but the smallest active regions.
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Title: Solar and Stellar Magnetic Activity
Authors: Schrijver, Carolus J.; Zwaan, Cornelis
2000ssma.book.....S Altcode:
Magnetic activity in the sun and similar stars results in a wealth of
phenomena--including starspots, nonradiatively heated outer atmospheres,
activity cycles, deceleration of rotation rates, and even, in close
binaries, stellar cannibalism. This volume uniquely combines studies of
the sun with those of other stars to provide a comprehensive picture of
stellar magnetic activity. Coverage brings together recent results in
solar and stellar studies to provide an illuminating, new view of the
subject. Key topics include radiative transfer, convective simulations,
dynamo theory, outer-atmospheric heating, stellar winds and angular
momentum loss. Researchers are provided with a state-of-the-art review
of this exciting field. Graduate students in astrophysics and plasma
physics will find the volume an ideal introduction to the subject. The
book will also interest researchers studying the connection of solar
Sctivity with the Earth's climate change, such as geologists, planetary
scientists and atmospheric scientists.
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Title: Solar and stellar magnetic activity.
Authors: Schrijver, C. J.; Zwaan, C.
2000CAS....34.....S Altcode:
This timely volume provides the first comprehensive review and synthesis
of the current understanding of the origin, evolution, and effects
of magnetic fields in the Sun and other cool stars. Magnetic activity
results in a wealth of phenomena - including starspots, nonradiatively
heated outer atmospheres, activity cycles, deceleration of rotation
rates, and even, in close binaries, stellar cannibalism - all of which
are covered clearly and authoritatively. This book brings together
for the first time recent results in solar studies, with their wealth
of observational detail, and stellar studies, which allow the study
of how activity evolves and depends on the mass, age, and chemical
composition of stars. The result is an illuminating and comprehensive
view of stellar magnetic activity. Observational data are interpreted
by using the latest models in convective simulations, dynamo theory,
outer-atmospheric heating, stellar winds, and angular momentum loss.
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Title: Phenomena in an Emerging Active Region. II. Properties of
the Dynamic Small-Scale Structure
Authors: Strous, Louis H.; Zwaan, Cornelis
1999ApJ...527..435S Altcode:
The magnetic flux emergence in growing active region NOAA 5617,
when it is about 8 hr old, shows an intricate fine structure. The
small-scale emergence events are characterized by a coincident
upflow and transient darkening (of about 2 Mm and 10 minutes) in the
continuum and line-center intensity followed by the appearance of one,
or in some cases two, new bright grains flanking the line-center
darkening. The bright grains (faculae) coincide with magnetic
flux concentrations and downflows. The footpoints move apart at on
average 1.4 km s<SUP>-1</SUP>. Flux emergence happens recurrently in
a number of locations widely distributed over the active region, which
appear to form a pattern with a wavelength of about 8 Mm. A preferred
orientation that fits Hale's polarity law is displayed by the spatial
pattern in the emergence locations, the emergence events themselves,
subsequent footpoint motion, and the Hα arch filament system. We find
long (~15 Mm) alignments of unipolar faculae of each magnetic polarity
that also follow the preferred orientation. We adapt the model for flux
emergence to accommodate the observed dynamic fine structure. Essential
new features are (1) the emerging bundle of flux tubes is frayed in
two systems, in vertical stacks, arranged in slightly curved, nearly
parallel sheets; and (2) many flux tubes emerge in multiple locations.
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Title: Studies of changing patterns of solar activity using the DRAO
Synthesis Radio Telescope.
Authors: Tapping, K.; Burke, I.; Cameron, H.; Harvey, K.; Zwaan, C.
1999JRASC..93Q.186T Altcode:
No abstract at ADS
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Title: Properties of Small-Scale Flux Emergence in a Young Active
Region
Authors: Strous, Louis; Zwaan, Cornelis
1999soho....9E..82S Altcode:
The magnetic flux emergence in growing active region NOAA 5617, when
it is about 8 hours old, shows an intricate fine structure. The
small-scale emergence events are characterized by a coincident
upflow and transient darkening (of about 2 Mm and 10 minutes) in the
continuum and line-center intensity, followed by the appearance of
one, or in some cases two, new bright grains flanking the line-center
darkening. The bright grains (faculae) coincide with magnetic flux
concentrations and downflows. The footpoints move apart at on average
1.4 km/s. Flux emergence happens recurrently in a number of locations
widely distributed over the active region, which appear to form
a pattern with a wavelength of about 8 Mm. A preferred orientation
which fits Hale's polarity law is displayed by the spatial pattern in
the emergence locations, the emergence events themselves, subsequent
footpoint motion, and the H-alpha arch filament system. We find long
(~ 15 Mm) alignments of unipolar faculae of each magnetic polarity
that also follow the preferred orientation. We adapt the model for flux
emergence to accommodate the observed dynamic fine structure. Essential
new features are: (1) the emerging bundle of flux tubes is frayed in
two systems; in vertical stacks, arranged in slightly curved, nearly
parallel sheets, and (2) many flux tubes emerge in multiple locations.
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Title: Coronal and chromospheric emission from cool stars in
near-simultaneous ROSAT all-sky survey and Mount Wilson data.
Authors: Piters, A. J. M.; Schrijver, C. J.; Schmitt, J. H. M. M.;
Rosso, C.; Baliunas, S. L.; van Paradijs, J.; Zwaan, C.
1997A&A...325.1115P Altcode:
Mt. Wilson Ca II H&K line-core emission fluxes for 215 F-,
G- and K-type stars were obtained within at most a few days of the
corresponding ROSAT All-Sky Survey observations. These stars cover wide
ranges of stellar activity, spectral type and luminosity class. In
this paper we study the well-known relationship between the Ca II
H&K line-core emission in excess of the minimum emission and the
soft X-ray emission. We find that flux densities normalised with the
bolometric flux densities are the best quantity in which to express
activity when comparing radiative emission in different temperature
regimes. We find a power-law relationship, in which the X-ray normalised
emission varies approximately quadratically with the normalised excess
Ca II H&K line-core emission. This relationship does not depend on
luminosity class at least up to luminosity class III, and it does not
depend on effective temperature. The scatter around this relationship
is consistent with the measurement errors. The X-ray spectral hardness
ratios of main-sequence stars increase with the X-ray flux densities;
a similar trend, but with substantially larger scatter, is also present
for evolved stars. A comparison between values from different passbands
of the Mt. Wilson HK spectrophotometer shows that relatively hot stars
((B-V)<=0.50) appear to have a Ca II line core emission peak about
a factor 2 to 3 wider than cooler stars.
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Title: VizieR Online Data Catalog: Near-simultaneous ROSAT and Mt
Wilson data (Piters+ 1997)
Authors: Piters, A. J. M.; Schrijver, C. J.; Schmitt, J. H. M. M.;
Rosso, C.; Baliunas, S. L.; van Paradijs, J.; Zwaan, C.
1997yCat..33251115P Altcode:
Table 1 lists near-simultaneous X-ray data and Ca II H&K line-core
emission data from the ROSAT All-Sky Survey and from the Mt. Wilson
H&K spectrometer, respectively. The stars in this sample are 215
bright F-, G-, and K-type stars. Table 2 lists the derived excess Ca II
H&K line-core and the X-ray flux densities for the same stars. (2
data files).
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Title: Why and Where do Filaments Form in Active Regions?
Authors: Gaizauskas, V.; Zwaan, C.
1997SPD....28.0249G Altcode: 1997BAAS...29..902G
New prominence models based on recent observations depend upon magnetic
reconnection between small-scale magnetic elements converging at a
polarity inversion (PI). How then to explain active-region filaments
where magnetic flux diverges over much of the lifetime of the region? A
partial answer is that still-growing active regions containing filaments
are not simple bipolar entities. They are instead multipolar activity
complexes (`sunspot nests') wherein magnetic flux can be compressed
along a meandering PI wherever new bipolar units emerge near old
ones. A complete answer requires particulars about the distribution
and motions of magnetic fields internal and external to the sunspot
nests. We therefore surveyed over 150 active regions photographed on
a large spatial scale at ORSO during 5 successive solar rotations in
1979, an epoch of rapid emergence and decay. Of the total number of
regions: - 5% are simple decaying bipolar plages with filaments on
the PI; - 5% are ambiguous cases with sometimes a filament and field
transition arches (FTA) sharing adjacent parts of a PI in a bipolar
plage; -70% have boundary filaments exterior to the concentrations of
magnetic flux around sunspots; - 61% are single bipoles of which 84%
have no internal filament on their PI; - 52% are activity complexes
(on at least one day, otherwise they are single bipoles) of which 60%
have one or more filaments inside the complex. We find that filaments
inside sunspot nests mark off bipolar entities from one another,
thus fulfilling the role of boundary filaments on the inside of the
nests. We conclude that the boundary filament is the quintessential
active- region filament. Examination of specific cases leads to the
further conclusion that force-free fields together with cancelling
flux play a critical role in forming boundary filaments.
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Title: A Dynamo Scenario|Observational Constraints on Dynamo Theory
Authors: Zwaan, Cornelis
1996SoPh..169..265Z Altcode:
In this paper, the term dynamo refers to the complex of
physical mechanisms that cause solar magnetic activity in all
its manifestations. Properties of that dynamo are inferred from
observational indications to fit them into a scenario. Properties and
models of the manifestations of strong magnetic field are briefly
summarized, together with their formation during the emergence of
ω-shaped loops from the toroidal flux system in the interface below
the convection zone. The evolution of magnetic concentrations and the
flux removal from the atmosphere, with indications for flux retraction,
are considered. Then the weak (INF) fields are discussed, together
with the role of upward floating LI- shaped loops in the removal of
toroidal flux. Finally features of strong and weak fields are fitted
into a scenario for a cyclic dynamo, in which the regeneration of the
poloidal field of proper sign relies on the cancellation of magnetic
flux in the surface transport interpreted as reconnection, followed
by retraction of reconnected loops.
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Title: Book reviews
Authors: Humphreys, R. M.; Kemp, S.; Savonije, G.; van der Hucht,
K. A.; van der Kruit, P. C.; Miley, G.; Bumba, V.; van Nieuwkoop,
J.; van Hoolst, T.; Cox, A.; Rutten, R. J.; Kleczek, J.; de Jager,
Cornelis; Jerzykiewicz, M.; Zwaan, C.; Poedts, S.; Sakai, Jun-Ichi;
Pecker, J. -C.; Heikkila, W.; de Jong, T.; Wilson, P. R.; Müller,
E. A.; Hoyng, P.; Icke, V.; Shore, S. N.; Achterberg, A.; Lucchin, F.;
Butcher, H.; Ne'Eman, Y.; Heidmann, J.; Belton, M. J. S.; de Graauw,
Th.; Waters, L. B. F. M.; Pacini, F.; Hultqvist, B.; Akasofu, S. -I.;
Vial, J. -C.; Schatzman, E.; van der Laan, H.; Cole, K. D.; Vanbeveren,
D.; Southwood, D.; van der Klis, M.; Katgert, Peter
1996SSRv...76..339H Altcode:
No abstract at ADS
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Title: Phenomena in an emerging active region. I. Horizontal dynamics.
Authors: Strous, L. H.; Scharmer, G.; Tarbell, T. D.; Title, A. M.;
Zwaan, C.
1996A&A...306..947S Altcode:
Horizontal dynamics in observations of NOAA AR 5617 are studied by
tracking individual elements through the field of view. Small magnetic
elements of both magnetic polarities occur everywhere in the active
region, and define unipolar thread-like concentrations of magnetic
field of up to 15Mm length. The horizontal granular flow field in
the active region is divergent (e-time scale 2.1hours) and clockwise
(time scale 32hours). Facular elements are tracers of (clumps of)
fluxtubes. A hierarchy of movement of magnetic elements appears:
Facular elements everywhere in the active region move obliquely toward
the edges of the active region of the same polarity as their own, faster
than those edges (as defined by strings of pores) move apart. The pores
move along the edges toward the major sunspots of their own polarity,
and the major sunspots of either polarity move apart. The separation
velocity of both polarities of facular elements is about 0.84km/s, of
pores about 0.73km/s, and that of the major sunspots is about 0.50km/s.
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Title: Patterns in the solar magnetic field (Invited Review)
Authors: Zwaan, C.; Harvey, K. L.
1994smf..conf...27Z Altcode:
No abstract at ADS
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Title: The Sun among the stars
Authors: Zwaan, C.
1994ASIC..433....3Z Altcode:
No abstract at ADS
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Title: Properties and Emergence Patterns of Bipolar Active Regions -
Part One
Authors: Harvey, Karen L.; Zwaan, Cornelis
1993SoPh..148...85H Altcode:
Patterns in the properties of bipolar active regions are determined
throughout Cycle 21. Active regions that emerged on the visible
hemisphere were identified on NSO/KP full-disk magnetograms during 29
solar rotations selected from 1975 through 1986. The bipolar active
regions are included only once in this sampling; their properties
are derived at the time of maximum development. In order to study
an unbiased sample over the entire range of areas larger than 2.5
square degrees (or 373 Mm<SUP>2</SUP>), their counts are corrected for
size-dependent effects that reduce the chance of their identifications.
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Title: Het wisselende aangezicht van zon en sterren.
Authors: Schrijver, K.; Zwaan, K.
1993Zenit..20..292S Altcode:
No abstract at ADS
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Title: Inzicht uit licht. (2) De spectrale streepjescode en nog wat.
Authors: Zwaan, C.
1993Zenit..20..132Z Altcode:
No abstract at ADS
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Title: Inzicht uit licht. 1. Kleuren en temperaturen van
hemellichamen.
Authors: Zwaan, C.
1993Zenit..20..106Z Altcode:
No abstract at ADS
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Title: Percolation theory and the geometry of photospheric magnetic
flux concentrations
Authors: Balke, A. C.; Schrijver, C. J.; Zwaan, C.; Tarbell, T. D.
1993SoPh..143..215B Altcode:
The magnetic field in solar active regions forms a highly structured
pattern without an apparent length scale. We study this pattern in
detail for a plage and its surroundings observed with the Swedish Solar
Observatory on La Palma. The magnetogram has a resolution of about
1/3″, after image optimisation. We analysed the geometric properties
of isolated patches of magnetic flux. Patches with a linear size up to
3″ appear to be statistically self-similar, with a fractal dimension
ofD<SUB>f</SUB> = 1.54 ± 0.05 for the relation between area and linear
size. This value agrees very well with the dimensionD<SUB>f</SUB>
= 1.56 which is found in percolation theory for clusters of tracers
placed randomly on a lattice with a tracer density below a critical
threshold. The distribution of observed cluster areas also agrees
with that of clusters on such a random lattice. The correspondence
between properties of observations and of clusters on randomly filled
lattices suggests that- well after emergence - the magnetic flux on
the Sun is randomly distributed at least up to sizes of about 3″
and possibly larger.
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Title: Rotation Rates of Active Nests on the Sun
Authors: van Driel-Gesztelyi, L.; van der Zalm, E. B. J.; Zwaan, C.
1993ASPC...46..504V Altcode: 1993mvfs.conf..504V; 1993IAUCo.141..504V
No abstract at ADS
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Title: X-ray/Optical Survey of Late-Type Stars
Authors: Piters, A. J. M.; Schmitt, J. H. M. M.; Schrijver, C. J.;
Baliunas, S.; Zwaan, C.; van Paradijs, J.
1993ASSL..183..377P Altcode: 1993pssc.symp..377P
No abstract at ADS
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Title: Magnetic struture in cool stars. XVIII. UV-line emission from
T Tauri stars.
Authors: Lemmens, A. F. P.; Rutten, R. G. M.; Zwaan, C.
1992A&A...257..671L Altcode:
The flux densities in the UV spectral emission lines from T Tauri
stars are compared with those from other cool stars. In the flux-flux
diagrams for pairs of UV emission lines the data points for T Tauri
stars extend the relations as defined by main-sequence and evolved stars
towards larger flux densities by a factor of 40. We discuss the large
emission-line flux densities and the relations between flux densities
from different lines in the light of magnetic activity and recent
models of accretion disks in T Tauri stars. The very large emission
fluxes in chromospheric lines may be explained by a large emitting
volume associated with the warm (about 10,000 K) inner disk boundary
layer, but the fluxes and the line broadening in the high-excitation
lines requires the presence of hotter material, with temperatures of
about 200,000 K, which must be attached to the inner disks but which
is not predicted by existing disk models.
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Title: Activity in Tidally Interacting Binaries
Authors: Schrijver, C. J.; Zwaan, C.
1992ASPC...26..370S Altcode: 1992csss....7..370S
No abstract at ADS
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Title: The Evolution of Sunspots
Authors: Zwaan, Cornelis
1992ASIC..375...75Z Altcode: 1992sto..work...75Z
The evolution of sunspots is described in the context of the evolution
of active regions. The nesting tendency in formation of sunspots is
pointed out. Birth and growth of an active region are attributed
to an Omega-shaped loop emerging from a deep-seated toroidal flux
strand. It is argued that the field strength in the toroidal strand
exceeds the equipartition value; the top of the Omega-loop, however
frayed, reaches the upper boundary of the convection zone at intrinsic
field strengths close to the local equipartition value. The formation
of pores is preceded by the convective collapse of flux tubes. The
coalescence of the buoyant tops of flux tubes leads to the growth
of large sunspots. Inclusions of bright structure in umbrae, and
the formation of dark cores are discussed. Penumbrae form after the
spot has reached a critical flux level. Most sunspots decay fast by
fragmentation. Spots decaying gradually are surrounded by moat cells,
across which magnetic features of both polarities are streaming away
from the spot; the decay rate appears to be approximately constant. The
removal of magnetic flux from the atmosphere, including the seeming
disappearance in situ, is addressed. Finally, the complementary roles
of Omega-loops and U-loops in the magnetic structure in convection
zone and atmosphere are discussed.
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Title: Active Nests on the Sun
Authors: van Driel-Gesztelyi, Lidia; van der Zalm, Ed B. J.; Zwaan,
Cornelis
1992ASPC...27...89V Altcode: 1992socy.work...89V
No abstract at ADS
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Title: Fractals in Magnetograms
Authors: Schrijver, C. J.; Zwaan, C.; Balke, A. C.; Tarbell, T. D.;
Lawrence, J. K.
1992ASPC...27...67S Altcode: 1992socy.work...67S
No abstract at ADS
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Title: Patterns in the photospheric magnetic field and percolation
theory
Authors: Schrijver, C. J.; Zwaan, C.; Balke, A. C.; Tarbell, T. D.;
Lawrence, J. K.
1992A&A...253L...1S Altcode:
The magnetic field in solar plages forms a highly structured pattern
with no apparent characteristic length scale. This pattern appears
to be a fractal with a dimension between 1.45 and 1.60. Small-scale
displacements of concentrations of magnetic flux in the network
are consistent with a random walk on a fractal with a similar
dimension. Percolation theory offers an effective explanation for
observed geometric properties of small-scale flux concentrations
in the solar photosphere, by demonstrating the close correspondence
with clusters formed by randomly placed tracers on a 2D (irregular)
lattice. Percolation theory also offers a model for the subdiffusive
behavior of tracers performing a random walk on clusters formed
by bonded sites. The geometry of flux concentrations and of the
displacement of magnetic flux as a function of time are equivalent
to situations in percolation theory below a critical value, called
'the percolation threshold'.
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Title: Magnetic structure in cool stars. XVII. Minimum radiative
losses from theouter atmosphere.
Authors: Rutten, R. G. M.; Schrijver, C. J.; Lemmens, A. F. P.;
Zwaan, C.
1991A&A...252..203R Altcode:
The emissions in several chromospheric and transition region lines and
in coronal soft X-rays are analyzed for a sample of cool stars. The
nature of the lower-limit flux densities is explored, and evidence is
given for the possibility of a basal, nonmagnetic heating mechanism
being responsible for these emission fluxes up to, and perhaps
including, the upper transition region. It is argued that the excess
flux density, derived by subtraction of the basal flux density from the
observed stellar flux, is the proper measure of magnetic activity. The
level of the basal flux density as a function of color is determined
to be 2 x 10 exp 6 erg/sq cm/s for F-type stars and 2 x 10 exp 5 erg/sq
cm/s for K-type stars.
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Title: Activity in tidally interacting binaries.
Authors: Schrijver, C. J.; Zwaan, C.
1991A&A...251..183S Altcode:
A data set encompassing 23 binaries for which essential properties
of components and orbits are known is presently used to study the
relationship between activity, rotation, and revolution in comparatively
close binaries, by comparison to the relationship defined by single
stars. Confirmation is obtained for previous reports that the
relationships between emissions from the outer stellar atmospheres
of the binaries are indistinguishable from those defined by single
stars. While the rotation-activity relationship for single stars can be
described in terms of fundamental stellar properties, properties of the
secondary are important in determining the activity of close binaries.
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Title: CA II H and K Measurements Made at Mount Wilson Observatory,
1966--1983
Authors: Duncan, Douglas K.; Vaughan, Arthur H.; Wilson, Olin C.;
Preston, George W.; Frazer, James; Lanning, Howard; Misch, Anthony;
Mueller, Jean; Soyumer, David; Woodard, L.; Baliunas, Sallie L.;
Noyes, Robert W.; Hartmann, Lee W.; Porter, Alain; Zwaan, Cornelis;
Middelkoop, Frans; Rutten, Rene G. M.; Mihalas, Dimitri
1991ApJS...76..383D Altcode:
Summaries are presented of the photoelectric measurements of stellar
Ca II H and K line intensity made at Mount Wilson Observatory during
the years 1966-1983. These results are derived from 65,263 individual
observations of 1296 stars. For each star, for each observing season,
the maximum, minimum, mean, and variation of the instrumental H and
K index 'S' are given, as well as a measurement of the accuracy of
observation. A total of 3110 seasonal summaries are reported. Factors
which affect the ability to detect stellar activity variations and
accurately measure their amplitudes, such as the accuracy of the H and K
measurements and scattered light contamination, are discussed. Relations
are given which facilitate intercomparison of 'S' values with residual
intensities derived from ordinary spectrophotometry, and for converting
measurements to absolute fluxes.
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Title: Magnetic Activity Across the Hertzsprung-Russell Diagram
(With 8 Figures)
Authors: Zwaan, C.
1991mcch.conf..241Z Altcode:
No abstract at ADS
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Title: Non-Steady Global Magnetic Fields in Kinematic Theory
Authors: van Geffen, J. H. G. M.; Hoyng, P.; Zwaan, C.
1991LNP...380..129V Altcode: 1991IAUCo.130..129V; 1991sacs.coll..129V
The dynamo equation for the mean field (B) contains a random forcing
term of unknown magnitude, which is therefore always omitted. The
influence of this term is potentially large. To evaluate its effect,
we employ ensemble averaging. If an ensemble average is used, there
is no random forcing term in the dynamo equation. The effect of
fluctuations is that the ensemble members get out of phase, so that
(B) 0. The damping time of (B) can be found by requiring that the mean
energy (BB) remains finite. The eigenvalues of the dynamo equation then
all have negative real parts. Im determines the period, and -Re/Im the
relative period stability of the dynamo. We have developed a code to
solve the equation for (BB) in a spherical shell (the convection zone),
assuming axisymmetry. We report our first results, which do not yet
include differential rotation. Using spherically symmetric boundary
conditions, we reproduce the well known 2-dynamo, whose behaviour is
known analytically. For instance, for an 2-dynamo located in a shell
with inner boundary at R/2, we find that (BB) remains finite for
R2/ = 1.48, where 3 represents turbulent diffusion and .y turbulent
vorticity. Taking = 1/4(βy)1/2 — a factor of four below maximum
helicity — implies that we have a dynamo number Ca ≡ Rα/β =
0.30. Using this value we find a damping time of 6 X 10-2 R 2/β
for (B), which is a measure for the coherence time of B in a single
ensemble member. This result implies that the large-scale field of
this particular α2-dynamo reorganizes its structure completely on
a time scale of only about one year (for solar values of R and β),
and it shows the enormous influence of random forcing in general.
---------------------------------------------------------
Title: Activity of Relatively Close Binaries
Authors: Schrijver, Carolus J.; Zwaan, Cornelis
1991LNP...380..435S Altcode: 1991sacs.coll..435S; 1991IAUCo.130..435S
Whereas the rotation-activity relationship for single cool stars can
be described in terms of fundamental stellar properties, the activity
in relatively close binaries is enhanced with respect to that of
single stars by an amount which depends strongly on the properties of
the companion. We consider mechanisms which could cause an excess in
activity of binaries, and conclude that it is likely that the presence
of a companion affects the interior structure of the stars in such
a way that either the efficiency of the dynamo or of the atmospheric
heating is enhanced.
---------------------------------------------------------
Title: Sunspot Nests as Traced by a Cluster Analysis
Authors: Brouwer, M. P.; Zwaan, C.
1990SoPh..129..221B Altcode:
The appearance of sunspot groups between August 1959 and December
1964 is investigated in search of sunspot nests. A sunspot nest is a
relatively small space on the surface within which a succession of
spot groups appears. A single-linkage clustering technique is used
to trace clusters in the three dimensions longitude, latitude and
time. The number of genuine sunspot nests is estimated and their
properties are determined.
---------------------------------------------------------
Title: Patterns of Activity
Authors: Zwaan, C.
1990PDHO....7...67Z Altcode: 1990dysu.conf...67Z; 1990ESPM....6...67Z
Patterns in the occurrence of bipolar active regions in the solar
atmosphere are discussed. This review focuses on the patterns in
the flux emergences, i.e., on patterns seen in young active regions
or in sunspots. The reason is that these may yield the most direct
information on the dynamo action in the convection zone. Particular
attention is paid to studies that try to describe quantitative aspects
of the patterns of activity.
---------------------------------------------------------
Title: Unresolved magnetic features: possibilities and limitations of
(I,V) diagnostics
Authors: Zwaan, C.
1989hsrs.conf..420Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic structure in cool stars. XVI. Emissions from the
outer atmospheres of M-type dwarfs.
Authors: Rutten, R. G. M.; Schrijver, C. J.; Zwaan, C.; Duncan, D. K.;
Mewe, R.
1989A&A...219..239R Altcode:
Consideration is given to emission from the outer atmospheres of M-type
dwarfs in several spectral lines originating from the chromosphere, the
transition-region, and the soft X-ray emission from the corona. It is
shown that M-type dwarfs systematically deviate from relations between
flux densities in soft X-rays and chromospheric and transition-region
emission lines. The quantitative relation between the equivalent width
of H-alpha and the Ca II, H, and K emission index is determined. It
is suggested that the emission in the Balmer spectrum may result from
back heating by coronal soft X-rays.
---------------------------------------------------------
Title: Relations between the Photospheric Magnetic Field and the
Emission from the Outer Atmospheres of Cool Stars. I. The Solar CA
II K Line Core Emission
Authors: Schrijver, C. J.; Cote, J.; Zwaan, C.; Saar, S. H.
1989ApJ...337..964S Altcode:
Observations of a solar active region complex and its surroundings are
used to establish a quantitative relation between the Ca II K line core
intensity and magnetic flux density. The Ca II K line core intensity
is transformed to a Ca II H + K line core flux density to facilitate
a comparison of solar and stellar data. A new absolute calibration for
the Mount Wilson Ca II H + K fluxes for G-type dwarfs is derived. The
minimum Ca II K flux, found in the centers of supergranulation cells
in quiet regions on the sun, is identical to the minimum flux that
is observed for solar-type stars. An expression is presented for the
nonlinear trend between the Ca II H + K line core excess flux density
and the absolute value of the magnetic flux density. Models that
explain the nonlinearity of the mean trend and the large intrinsic
scatter about it are discussed. The solar data define a relation that
is similar to the relation between stellar hemisphere-average magnetic
flux densities and Ca II H + K excess flux densities.
---------------------------------------------------------
Title: Asynchronous rotation in close binary systems with circular
orbits.
Authors: Habets, G. M. H. J.; Zwaan, C.
1989A&A...211...56H Altcode:
The origin of asynchronism observed in some binary systems in which
the components rotate much more rapidly or much more slowly than the
orbital rate was investigated using Pylyser's (1988) evolutionary models
of stars of 1.0, 1.5, and 2.0 solar masses. Under the assumption of
rigid-body rotation, the evolution of the angular rotation velocity
from the main sequence up to the red-giant branch is derived; the
orbital circularity of these systems is explained by strong tidal
interaction, either during the early premain-sequence phase, or, for
systems containing a white dwarf, during the red-giant phase of its
progenitor. It is argued that, at the zero-age main sequence (ZAMS),
the binary components rotate much faster than the orbital rate. During
the evolution off the ZAMS, the angular rotation rate of the components
decreases, mainly because of the increase of the moment of inertia,
and, possibly, because of magnetic braking.
---------------------------------------------------------
Title: SYSTEMATIC OBSERVATIONS OF THE SUN (In honour of Helen Dodson
Prince): Future work
Authors: Livingston, W.; Zwaan, K.; Hiei, E.; Paterno, L.
1989HiA.....8..677L Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book Review: Small-scale magnetic flux concentrations in the
solar photosphere. / Vandenhoeck & Ruprecht, Gottingen, 1986
Authors: Zwaan, C.
1988SSRv...48..187Z Altcode: 1988SSRv...48..187D
No abstract at ADS
---------------------------------------------------------
Title: Joint Discussion on Topics of Sessions 5 and 6
Authors: Zwaan, C.; Maltby, P.
1987rfsm.conf..165Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Elements and patterns in the solar magnetic field.
Authors: Zwaan, Cornelis
1987ARA&A..25...83Z Altcode:
This review concentrates on observational studies that may reveal the
MHD processes in the solar interior and photosphere that generate
and shape the magnetic field. The intrinsically strong magnetic
field is contained in seemingly isolated elements, ranging from the
thick boundles of flux tubes in sunspots to the hypothetical thin
flux fibers. These elements are arranged in the typical patterns
observed in active regions and in the magnetic network. The processes
of emergence of magnetic flux into the atmosphere and removal of flux
from the photosphere are directly related to the magnetic structure
and dynamics in the solar convective envelope.
---------------------------------------------------------
Title: Sunspot Nests - Manifestations of Sequences in Magnetic
Activity
Authors: Castenmiller, M. J. M.; Zwaan, C.; van der Zalm, E. B. J.
1986SoPh..105..237C Altcode:
For the period August 1959-December 1964 the Greenwich Photoheliographic
Results were searched for sunspot nests. Such a nest is a sequence of
sunspot groups that appear within a small area on the solar surface
and that last for several months. The search procedure is described and
data for 41 probable sunspot nests are given. At least three quarters
of these nests appear to be real, and not chance clusters.
---------------------------------------------------------
Title: Relations Between Magnetic Activity and Stellar Properties
Authors: Zwaan, Cornelis
1986LNP...254...19Z Altcode: 1986csss....4...19Z
This paper discusses the basal emission found in strong chromospheric
resonance lines, the activity emission, and the tight power-law
relations that exist for the majority of cool stars between the flux
densities measured in the various coronal, transition-region and excess
chromospheric emissions (the "flux-flux relations"). These relations
do not depend on effective temperature and surface gravity.
---------------------------------------------------------
Title: The Emergence of Magnetic Flux
Authors: Zwaan, C.
1985SoPh..100..397Z Altcode:
This paper first summarizes the morphology and dynamics of emerging
flux regions and arch filament systems and then discusses detailed
observations of a particular active region with emerging magnetic flux.
---------------------------------------------------------
Title: The Bright Pre-main Sequence Variable HR5999
Authors: The, P. S.; Tjun-A-Djie, H. R. E.; Brown, A.; Catala, C.;
Doazan, V.; Linsky, J.; Mewe, R.; Praderie, F.; Talavera, A.; Zwaan, C.
1985IrAJ...17...79T Altcode:
A meeting to review the reduced coordinated observations by EXOSAT, IUE
and ground-based instruments of the bright and variable Herbig A7e star
HR 5999 is summarized. The attempt was to delineate the relationship
between the various observable quantities of its atmosphere (colors,
emission-line fluxes, wind velocities, etc.) in order to develop a
dynamical model of the extended atmosphere. The observations were made
around September 11, 1983.
---------------------------------------------------------
Title: A study of ultraviolet and X-ray emissions of selected
solar regions
Authors: Schrijver, C. J.; Zwaan, C.; Maxson, C. W.; Noyes, R. W.
1985A&A...149..123S Altcode:
Apollo Telescope Mount-based Skylab observations of quiet regions,
coronal holes, and active regions of various sizes and in various stages
of evolution are studied. Total intensities in coronal emissions are
noted to increase more rapidly than linearly with total intensities in
chromospheric emissions, and areas of active regions determined from
chromospheric emissions are found to be systematically larger than
areas determined from transition region emissions. The coronal X-ray
temperature, as well as the average intensities of active regions,
depend on the area of the active region, on Ca II intensity, and on
relative coverage by sunspots.
---------------------------------------------------------
Title: Magnetic structure in cool stars. VIII. The MG II H and K
surface fluxes in relation to the MT Wilson photometric CA II H and
K measurements.
Authors: Oranje, B. J.; Zwaan, C.
1985A&A...147..265O Altcode:
Data from IUE observations of 14 F, G, and K stars of luminosity
II-V in the Mg II h + k lines are presented in tables and compared
with published Mt. Wilson photometry of the Ca II H + K lines, and
empirical relations are derived to facilitate the use of Ca II H + K
data in investigating the chromospheric structure of cool stars. The
results are presented in graphs, and consideration is given to the
Vaughan-Preston gap for main-sequence stars; the (B-V)-dependent
flux minima for main-sequence (LC V) stars, LC IV subgiants, and
LC III giants (consistent with a dynamo model with magnetic braking
and tidal synchronization); the very large fluxes of FK Com stars;
and the positions of the LC II bright giants and LC I supergiants on
the chromospheric-flux/color diagram.
---------------------------------------------------------
Title: High Resolution Spectroscopy of Active Regions - Part One -
Observing Procedures
Authors: Zwaan, C.; Brants, J. J.; Cram, L. E.
1985SoPh...95....3Z Altcode:
We describe an observing program designed to obtain spectra of sunspots,
pores, and other features in active regions using the Vacuum Tower
Telescope and Echelle Spectrograph at Sacramento Peak Observatory. The
spectral lines used in this study have been especially chosen to allow
pointed studies of fine structure in the intensity distribution,
and in the velocity and magnetic fields in the photospheric levels
of active regions, and to relate this structure to chromospheric
observations made in the CaII H line. We demonstrate the capacities
of the observing program by two examples: umbral fine structure, and
an emerging active region. Although the umbral spectrograms resolve
the brightness structure down to less than one arc sec we do not
find clear-cut relations in the spatial variations of brightness,
magnetic field strength and line-of-sight velocity across the umbral
structure. In the emerging active region several processes are observed
in their relationship: the rising and draining of an arch-filament
system, the convective collapse of flux tubes, and the growth of a
sunspot pore by coalescing fluxtubes.
---------------------------------------------------------
Title: Outer Atmospheres of Evolved Starss of Low Activity
Authors: Zwaan, C.
1985iue..prop.2390Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Atmospheric Fine Structure as a Probe for the Solar Interior
Authors: Zwaan, C.
1985LNP...233..263Z Altcode: 1985hrsp.proc..263Z
The study of the solar atmosphere's fine structure features promises
to deepen insight into the solar interior. Attention is given to the
use of magnetic elements as velocity field tracers, the large scale
structures of the solar interior's magnetic field, emerging flux
regions, the removal of magnetic flux, and the structure and evolution
of magnetic elements. The importance of efforts in site testing and
high resolution instrumentation development studies is stressed.
---------------------------------------------------------
Title: The Interacting Binary HD 352 (5 CET)
Authors: Zwaan, C.
1984iue..prop.2071Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Book reviews
Authors: van der Hucht, K. A.; Pedersen, A.; Kesák, Ľ.; Zwaan,
C.; Fárník, F.; Kovalevsky, J.; van Beek, H. F.; Mewe, R.; Page,
D. E.; van Genderen, A. M.; Koch-Miramond, L.; de Jager, C.; Ooms,
G.; Pedersen, Arne; de Jager, C.; Hultqvist, Bengt; Waters, R.;
Schrijver, J.
1984SSRv...38..179V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric Transition Region and Coronal Diagnostics for
Stellar Magnetic Activity
Authors: Schrijver, C. J.; Zwaan, C.
1984srps.conf..291S Altcode:
Using various diagnostics magnetic activity has been inferred for the
vast majority of stars with convective envelopes. The authors discuss
relations between emissions from different temperature regimes, and
their dependence on various stellar parameters. They suggest several
practical indicators for stellar magnetic structure and acitivity,
and outline a few topics for which space observations would be useful.
---------------------------------------------------------
Title: Book-Review - Activity in Red Dwarf Stars
Authors: Byrne, P. B.; Rodono, M.; Zwaan, C.
1984SSRv...38..180B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Patterns of the Sun
Authors: Gaizauskas, V.; Harvey, K. L.; Harvey, J. W.; Zwaan, C.
1983S&T....66..291G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Large-scale patterns formed by solar active regions during
the ascending phase of cycle 21
Authors: Gaizauskas, V.; Harvey, K. L.; Harvey, J. W.; Zwaan, C.
1983ApJ...265.1056G Altcode:
Synoptic maps of photospheric magnetic fields prepared at the Kitt Peak
National Observatory are used in investigating large-scale patterns
in the spatial and temporal distribution of solar active regions
for 27 solar rotations between 1977 and 1979. The active regions are
found to be distributed in 'complexes of activity' (Bumba and Howard,
1965). With the working definition of a complex of activity based
on continuity and proximity of the constituent active regions, the
phenomenology of complexes is explored. It is found that complexes of
activity form within one month and that they are typically maintained
for 3 to 6 solar rotations by fresh injections of magnetic flux. During
the active lifetime of a complex of activity, the total magnetic flux
in the complex remains steady to within a factor of 2. The magnetic
polarities are closely balanced, and each complex rotates about the
sun at its own special, constant rate. In certain cases, the complexes
form two diverging branches.
---------------------------------------------------------
Title: Chromospheric and coronal indicators of stellar magnetic
structure
Authors: Zwaan, C.
1983IAUS..102...85Z Altcode:
The use of observations of the H and K lines of Ca II, UV emission
lines, and soft X-rays to characterize the magnetic structure of F, G,
and K stars is discussed, and representative data are summarized. The
solar data are reviewed, and stellar data are presented in graphs. Close
relations between the soft-X-ray flux, the transition-region flux,
and the chromospheric or H+K flux are identified and interpreted,
and a need for simultaneous magnetographic observations is indicated.
---------------------------------------------------------
Title: Empirical NLTE analyses of solar spectral lines. IV - The Fe
I curve of growth
Authors: Rutten, R. J.; Zwaan, C.
1983A&A...117...21R Altcode:
The effects of departures from local thermodynamic equilibrium on
the equivalent widths of solar Fe I lines are studied as an example
for the analysis of the stellar curve of growth. The solar curve of
growth obtained is based on the nonlocal thermodynamic equilibrium
(NLTE) modeling of the solar spectrum of Lites (1972, 1973) and the
best available oscillator strengths for 991 Fe I lines. The empirical
curve obtained relating the equivalent width-wavelength ratio to the
oscillator strength under NLTE is shown to differ appreciably from
curves neglecting the NLTE ionization departures, although these effects
may be corrected by assuming a NLTE-masking model. Theoretical NLTE
curves of growth are also presented, and splittings due to wavelength
dependency, differences in NLTE excitation, and variation in collisional
damping, which are largely hidden by noise in observed values, are
discussed. A new value for the solar iron abundance of 0.000047 times
the hydrogen abundance is also derived.
---------------------------------------------------------
Title: Observational Requirements - Ground-Based - Solar and Stellar
Magnetic Fields
Authors: Zwaan, C.
1983IAUS..102..503Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal activity in F-, G-, and K-type stars - Empirical
relations between stellar parameters
Authors: Mewe, R.; Schrijver, C. J.; Gronenschild, E. H. B. M.;
Zwaan, C.
1983IAUS..102..205M Altcode:
For a sample of 58 late type stars the authors analyse the relations
between the soft X-ray flux density F<SUB>x</SUB>, the Ca II H and
K line-core flux density F<SUB>H+K</SUB>, and parameters determining
the global stellar structure. By analysing the soft X-ray spectra from
15 stars the authors determine the coronal temperatures T and specific
emission measures per unit area ζ. They discuss the dependence of T on
B-V, F<SUB>x</SUB> and stellar radius R. The diagram of the specific
emission measure ζ against the temperature T is interpreted in terms
of a coronal model consisting of static loops. Also, a search for time
variations in the X-ray flux has been performed.
---------------------------------------------------------
Title: Book reviews
Authors: Ne'Eman, Y.; Zahn, J. -P.; Habing, H. J.; Wittenberg, H.;
Zwaan, C.; Murray, C. A.; de Jager, Cornelis; Kresák, L.
1982SSRv...33..459N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Structure of Sunspots - Part Four - Magnetic Field
Strengths in Small Sunspots and Pores
Authors: Brants, J. J.; Zwaan, C.
1982SoPh...80..251B Altcode:
Magnetic field strengths in small umbrae and pores are measured using
the line Ti I λ6064.6 Å, which is formed purely in umbrae. We
find field strengths between 1900 and 2600 G in the darkest parts
of small umbrae and of well established pores; the spread is partly
intrinsic. The field strength in diffuse transient `protopores'
amounts to 1500 ± 250 G.
---------------------------------------------------------
Title: Magnetic structure in cool stars. V. Chronospheric and
transition-region emission from giants.
Authors: Oranje, B. J.; Zwaan, C.; Middelkoop, F.
1982A&A...110...30O Altcode:
The outer atmospheres of giants at larger heights than the low
chromosphere, where the Ca II H and K lines are formed, are studied. In
particular, the coupling between the upper chromosphere and the
transition region is investigated. G and early K-type giants of
different Ca II H and K line-core flux were selected for observations
with the IUE. The transition-region flux, defined as the sum of
fluxes in the lines of Si IV, C IV, and N V are plotted against the
chromospheric flux, defined as the sum of the fluxes in lines of O I and
Si II. A surprisingly tight relation is found between transition-region
and chromospheric flux which extends over two orders of magnitude
in the chromospheric flux and three orders in the transition-region
flux. This relation holds for all stars in the sample, including giants,
main-sequence stars, and close binaries. The results are discussed in
terms of discrete solar-like magnetic structure.
---------------------------------------------------------
Title: Coronal activity in F-, G-, and K-type stars; relations between
parameters characterizing stellar structures and X-ray emission
Authors: Schrijver, C. J.; Mewe, R.; Zwaan, C.
1982AdSpR...2i.243S Altcode: 1982AdSpR...2..243S
A sample of 52 stars containing dwarfs and giants is subjected to
a multidimensional factor analysis. The parameters used are the
soft X-ray flux at the stellar surface F<SUB>x</SUB>, the Ca II H+K
line-core flux F<SUB>H+K</SUB>, the stellar radius and mass. We find
a high correlation between F<SUB>x</SUB> and the Ca II H+K excess flux
ΔF<SUB>H+K</SUB> obtained by subtracting an observational lower-limit
flux from F<SUB>H+K</SUB>. We conclude that the lower-limit Ca II flux
is uncorrelated with the stellar X-ray emission. The common-factor
analysis shows that, for the present sample, F<SUB>x</SUB> depends
only on ΔF<SUB>H+K</SUB>, and not on the stellar radius or mass. All
stars included in our analysis follow the relation F<SUB>x</SUB>
~ Δ<SUP>1.4</SUP><SUB>H+K</SUB> over almost four decades in
F<SUB>x</SUB>.
---------------------------------------------------------
Title: Magnetic Structure of F, G, and K Type Stars, II
Authors: Zwaan, C.
1982iue..prop.1251Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The MHD of sunspots. Invited reviews presented at the Joint
Meeting of IAU Commissions 10, 12, and 44 during the IAU Assembly
in Montreal, August 20, 1979.
Authors: Zwaan, C.
1981SSRv...28..387Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Magnetohydrodynamics of Sunspots
Authors: Zwaan, Cornelis
1981SSRv...28..385Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic structure in cool stars. I. The CA II H and K emission
from giants.
Authors: Middelkoop, F.; Zwaan, C.
1981A&A...101...26M Altcode:
Estimates of the intensities of the Ca II H and K emission lines of 500
subgiants, giants and supergiants are reanalyzed in an investigation
of the emission mechanisms in evolved cool stars. The Ca II emission
intensity classes estimated by Wilson (1976) are compared with B-V
colors of the stars in the sample. The majority of K- and M-type giants
and many late G-type giants and subgiants are found to constitute
a band of intensity class increasing with B-V, while G-type giants
show a large spread in intensity class. Enhanced H and K emission
is found in two groups of evolved stars: some of the G-type giants,
and short-period binaries among the giants and subgiants with orbital
periods shorter than a critical value, and the emission is explained
by a relatively large stellar rotation rate. Data are consistent with
the hypothesis that Ca II H and K emission depends on dynamo action
in the convective envelope, with dynamo efficiency decreasing with
decreasing rotation rate.
---------------------------------------------------------
Title: Magnetic structure in cool stars. II - Observational evidence
for transverse magnetic fields
Authors: Tinbergen, J.; Zwaan, C.
1981A&A...101..223T Altcode:
The apparently intrinsic linear polarization of the light from nearby
F, G, K and M stars is attributed to transverse magnetic fields near
the stellar limb in activity belts on either side of the stellar
equator. The polarization results from the pi components of strong
spectral lines being more saturated than sigma components (Leroy,
1962). The proposed mechanism is compatible with existing observational
data. Stringent tests and prospects for further observations are
indicated.
---------------------------------------------------------
Title: Magnetic braking in low-mass X-ray binaries.
Authors: Verbunt, F.; Zwaan, C.
1981A&A...100L...7V Altcode:
It is pointed out that the cool main-sequence components of cataclysmic
binaries and low-mass X-ray binaries will undergo rotational braking
by a magnetically coupled stellar wind, in a way similar to single
main-sequence stars. Since the low-mass components of close binaries
are forced by tidal forces to remain in corotation, this leads
to a loss of orbital angular momentum from the system, and to an
enhanced mass-transfer rate. Using the empirical braking law for
cool main-sequence stars, it is found that in systems with secondary
mases equal to, or greater than 0.3 to 0.5 solar masses the induced
mass-transfer rate is larger than that produced by gravitational
radiation losses. For a secondary mass of 0.8 solar masses, a mass
transfer rate of 10 to the -8.5th is achieved, sufficient to power
the brightest low mass X-ray binaries.
---------------------------------------------------------
Title: Solar magnetic structure and the solar activity cycle. Review
of observational data.
Authors: Zwaan, C.
1981NASSP.450..163Z Altcode: 1981suas.nasa..163Z
Data on solar magnetism that may offer clues for understanding
stellar magnetism in general were reviewed. Magnetic phenomena in the
photosphere and low chromosphere, where the magnetic structure can be
studied in detail, are discussed. Properties of the discrete magnetic
elements and their extensions through the chromosphere and transition
zone up to the corona are described. The structure and evolution of
active regions and other structured aggregates of magnetic elements
are considered.
---------------------------------------------------------
Title: Results from an extensive Einstein stellar survey.
Authors: Vaiana, G. S.; Cassinelli, J. P.; Fabbiano, G.; Giacconi,
R.; Golub, L.; Gorenstein, P.; Haisch, B. M.; Harnden, F. R., Jr.;
Johnson, H. M.; Linsky, J. L.; Maxson, C. W.; Mewe, R.; Rosner, R.;
Seward, F.; Topka, K.; Zwaan, C.
1981ApJ...245..163V Altcode:
The preliminary results of the Einstein Observatory stellar X-ray survey
are presented. To date, 143 soft X-ray sources have been identified with
stellar counterparts, leaving no doubt that stars in general constitute
a pervasive class of low-luminosity galactic X-ray sources. Stars along
the entire main sequence, of all luminosity classes, pre-main sequence
stars as well as very evolved stars have been detected. Early type
OB stars have X-ray luminosities in the range 10 to the 31st to 10 to
the 34th ergs/s; late type stars show a somewhat lower range of X-ray
emission levels, from 10 to the 26th to 10 to the 31st ergs/s. Late type
main-sequence stars show little dependence of X-ray emission levels upon
stellar effective temperature; similarly, the observations suggest weak,
if any, dependence of X-ray luminosity upon effective gravity. Instead,
the data show a broad range of emission levels (about three orders of
magnitude) throughout the main sequence later than F0.
---------------------------------------------------------
Title: The Size Dependence of Contrasts and Numbers of Small Magnetic
Flux Tubes in an Active Region
Authors: Spruit, H. C.; Zwaan, C.
1981SoPh...70..207S Altcode:
Intensity contrasts and number densities of bright points, knots and
pores ranging in size between 0″.15 and 4″ are studied using high
resolution pictures in Mg b<SUB>1</SUB> of a young active region. On the
average, the contrast in the wing of the line increases very strongly
with decreasing size, while the continuum contrast increases more
slowly. The ratio of contrast in the line to contrast in the continuum
increases rapidly with decreasing size. The possibility is explored of
using this contrast ratio as an indicator of size. The distribution
of the contrast ratio in a part of the active region is used in this
way to derive a size distribution of facular points. The resulting
distribution has a limited accuracy, but is free from systematic
distortion due to selection effects. Validity checks on the method are
presented. We measure the size distribution of the pores in the same
area, and combine the result with that for the facular points. The
combined distribution shows that the surface area covered by magnetic
elements with diameter δ has a maximum near δ = 0″.8. It increases
roughly proportional to δ for δ < 0″.3 and falls off as δ for
δ > 1″.5. It is inferred that elements with 0″.5 < δ <
1″.6, which show no conspicuous contrast in the line wing or in the
continuum, occupy as much area as the pores, and twice as much as the
facular points. We suggest that the changing appearance of a facular
area with increasing height of formation reflects at least as much
the increasing weight of the small elements in the contrast as a real
change in intrinsic properties (such as the diameter) of individual
elements. A spatial resolution better than 0″.1 may be needed to
resolve the individual elements in plages and the chromospheric network.
---------------------------------------------------------
Title: Large-Scale Patterns in Solar Activity During the Ascending
Phase of Cycle 21
Authors: Gaizauskas, V.; Harvey, K.; Harvey, J.; Zwaan, C.
1981BAAS...13R.906G Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Coronal Activity an F-Type G-Type and K-Type Stars
Authors: Mewe, R.; Schrijver, C. J.; Zwaan, C.
1981SSRv...30..191M Altcode:
Soft X-ray (0.3 3.5 keV) observations with the Imaging Proportional
Counter (IPC) onboard Einstein Observatory are presented for a sample
of some 20 cool stars of luminosity classes III V. The results are
compared with the Ca II H and K emission, which had served as a
selection criterion. The specific X-ray flux F<SUB>X</SUB> is an
increasing function of the specific Ca II H and K line-core flux
F<SUB>H+K</SUB>. This correlation can be considerably improved by
replacing F<SUB>H+K</SUB> by the excess flux (ΔF<SUB>H+K</SUB>)
above a certain lower limit which varies with B-V. This relation
holds with little scatter over the two decades in F<SUB>X</SUB> in
our sample. The F<SUB>X</SUB>-ΔF<SUB>H+K</SUB> relation shows no
significant dependence on spectral type or luminosity class, it suits
close binaries as well as single stars. However, the coronal X-ray
temperature T<SUB>c</SUB> strongly depends on the luminosity class:
T<SUB>c</SUB>≈ 3 10<SUP>6</SUP> K for dwarfs and ≈ 10<SUP>7</SUP>
K for giants. The results are interpreted in the framework of magnetic
activity. The X-ray emission and the excess Ca II H and K flux are
attributed to magnetic structure in the corona and chromosphere,
the magnetic features emerging from the stellar convective envelope,
where they are generated by dynamo action.
---------------------------------------------------------
Title: Empirical sunspot models - Statement of the problem
Authors: Zwaan, C.
1981phss.conf..123Z Altcode:
The motivations for devising empirical sunspot models and possible
data input to them are considered. Use of empirical models is noted to
place greater constraints on the physical processes which determine
the atmospheric structure than observational data does. Key factors
which must be included in any sunspot model comprise the thermodynamic
parameters, the magnetic field, and the velocity field. Choice of
a suitable coordinate system for the key factors is discussed, with
suggestions made of the vertical and radial directions, size dependence,
and separate calculations for the umbra and penumbra. An assumption
of hydrostatic equilibrium is recommended for the photosphere and the
low chromosphere. Problems extant in deriving models of the umbral
photosphere from the visible spectrum are outlined.
---------------------------------------------------------
Title: Magnetic Activity in Cool Stars
Authors: Zwaan, C.
1981MitAG..51..139Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic Structure of F, G, and K Type Stars
Authors: Zwaan, C.
1981iue..prop..962Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic activity in cool stars
Authors: Zwaan, C.
1981AGAb...51..139Z Altcode:
The solar magnetic structure is discussed as a framework for stellar
magnetism. Ways of observing magnetic structure in cool stars are
outlined with emphasis on observations of Ca 2, H, and K emission
in main sequence stars. It is inferred that the chromospheric,
transition-region, and coronal emissions are correlated manifestations
of magnetic activity in stars with convective envelopes, and that the
main level of activity depends on the rotation rate.
---------------------------------------------------------
Title: Magnetic field strengths in umbrae and pores
Authors: Zwaan, C.; Brants, J. J.
1981phss.conf..210Z Altcode:
A technique for accurately determining the magnetic field strength in
small umbrae and pores through use of line spectra which are enhanced in
the umbral spectrum is described. An example is provided of the field
strength obtained from taking the difference in the wavelength between
the medians of the line cores (yielded by an echelle spectrograph) for
densitometer tracings in the direction opposite to polarization. The
lines comprised Ti I at 6064.6 A, and additional readings of Fe I at
6173.3 A and Fe I at 6302.5 A. The field strength determined from the
photospheric Fe lines is shown to be less than that from the umbral
atmosphere Ti line, which is caused by the Fe sigma components being
strongly blended into the central component, which is weak in the Ti
line. Finally, it is demonstrated that the strength of the field is
independent of the size of the spot.
---------------------------------------------------------
Title: An emerging active region - Some preliminary results
Authors: Brants, J. J.; Zwaan, C.; Cram, L. E.
1981phss.conf...60B Altcode:
Observations of an emerging flux region on July 24, 1979 are
analyzed in terms of six spectral lines, the broadband continuum,
and the Ca II K line core. The process of formation of the umbrae,
including the appearance of protopores leading to fully formed pores,
is described. Results are presented from spectrographic observation of
the 6302 A and 5692 A lines, taken while pores were still forming. Field
strengths of 1900 and 2500 gauss were detected, in line with previous
measurements of pore groups.
---------------------------------------------------------
Title: Stellar magnetic structure and activity.
Authors: Zwaan, C.
1981ASIC...68..463Z Altcode: 1981spss.conf..463Z
Observational aspects of stellar magnetism are summarized. Methods
of observing stellar magnetic structure and activity are discussed,
including search for sigma components in spectral lines of large
magnetic splitting, broadband, spectral, and line photometry. Ca II
H and K emissions are discussed for main sequence stars, giants,
and spectroscopic binaries. Relationships are analyzed between
emissions from chromospheres, transition regions, and coronas, and
evolutionary scenarios are described for single stars and short-period
binaries. Long- and short-period variations of magnetic activity
are discussed. At the outset, four hypotheses are stated which give
structure to the following discussion and suggest empirical tests.
---------------------------------------------------------
Title: Methods for the analysis of stellar spectra veiled by lines
(III).
Authors: Greve, A.; Zwaan, C.
1980A&A....90..239G Altcode:
UV spectrograms of the sun and of Alpha CMi are analyzed to investigate
the effects of finite spectral resolution in the interpretation
of stellar spectra veiled by lines. The recorded completeness of
intrinsic spectral detail is examined by means of the ruggedness
parameter of Ivanov and Salman-Zade (1976). Solar spectrograms of
approximately 0.03 A resolution are a good representation of the true
spectral distribution. The power of the Minnaert-Houtgast method for
tracing line wings and establishing the local 'continuum' background
in spectra crowded with lines is demonstrated.
---------------------------------------------------------
Title: A Polarimeter for a Fourier Transform Spectrometer and Initial
Solar Observations
Authors: Harvey, J.; Brault, J.; Stenflo, J.; Zwaan, C.
1980BAAS...12R.476H Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Chromospheric and Coronal Activity in F-Type G-Type and
K-Type Stars
Authors: Mewe, R.; Zwaan, C.
1980SAOSR.389..123M Altcode: 1980csss....1..123M
No abstract at ADS
---------------------------------------------------------
Title: Chromospheres
Authors: Zwaan, C.
1980iue..prop..553Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Report from the discussion group on a large optical
observational facility (LEST)
Authors: Zwaan, C.
1980fsoo.conf..292Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: MHD phenomena in the photosphere.
Authors: Zwaan, C.
1979ssms.conf...71Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Report from the discussion group on a large optical
observational facility (LEST).
Authors: Zwaan, C.
1979MmArc.106..295Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Appearance of Magnetic Flux in the Solar Photosphere
Authors: Zwaan, Cornelis
1978SoPh...60..213Z Altcode:
Ideas and models for the appearance of photospheric magnetic
structure are confronted with observational data. Some findings are:
The magnetic flux emerging in an active region consists of a bundle
of flux tubes which were already concentrated before penetrating into
the photosphere. A model of a rising bunch of flux tubes joining into
a few strands at larger depths describes the coalescence of spots near
the leading and following edges of the active region while more flux
may surface near the center of the region. There is no observational
evidence for appreciable helical twists in the flux bundles.
---------------------------------------------------------
Title: Report from the discussion group on a large optical
observational facility (LEST)
Authors: Zwaan, C.
1978fsoo.conf..292Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetic structures in photosphere of sun and stars
Authors: Zwaan, C.
1977MmSAI..48..525Z Altcode:
Following a review of the principal observational data on the magnetic
structures in the solar photosphere and low chromosphere, some simple
models are examined which provide a semi-quantative interpretation of
the observed structures. The interplay between the magnetic field and
the velocity fields (rotation, convection), is examined, along with
its role in the emergence and redistribution of magnetic flux over
the solar surface. The magnetic structure to be expected in stars with
convective zones is studied, along with the observational indications
for stellar magnetic activity.
---------------------------------------------------------
Title: Fotograferen van zonnevlekken met een 17-cm refractor.
Authors: Brückner, W.; Zwaan, C.
1976Zenit...3...13B Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On Differences in Differential Rotation
Authors: Vantend, W.; Zwaan, C.
1976IAUS...71...45V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Solar activity (Activité solaire).
Authors: Newkirk, G.; Dunn, R. B.; Mehltretter, P.; MacQueen, R.;
Bonnet, R. M.; White, O. R.; Fokker, A. D.; Zwaan, C.; Bruzek, A.;
Durrant, C.; Grossmann-Doerth, U.; Mehltretter, J. P.; Svestka, Z.;
de Feiter, L. D.; Tandberg-Hanssen, E.; Howard, R.; Stix, M.; Pneuman,
G. W.; Hundhausen, A. J.; Sawyer, C.; Simon, P.
1976IAUTA..16b..13N Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Sunspots
Authors: Zwaan, C.
1976IAUTA..16...24Z Altcode: 1976IAUT...16A..24Z
No abstract at ADS
---------------------------------------------------------
Title: Structure of Sunspots. III: A Minimum-Gradient Model Atmosphere
for Umbrae
Authors: Zwaan, C.
1975SoPh...45..115Z Altcode:
We present a minimum-gradient model to complement a maximum-gradient
model (Zwaan, 1974). Both models are compatible with published
continuum intensities and within constraints on the temperature
gradient; both are adjusted to T<SUB>eff</SUB>=4000K. In order to
explain the visual intensities the minimum-gradient model requires an
additional `line-haze' opacity which is only slightly smaller than in
the maximum-gradient model.
---------------------------------------------------------
Title: Proceedings of the First European Solar Meeting. Florence,
February 25 - 27, 1975.
Authors: Chiuderi, C.; Landini, M.; Righini, A.; Zwaan, C.
1975MmArc.105....1C Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Structure of Sunspots. II: A Continuum Model Atmosphere for
Dark Umbral Cores
Authors: Zwaan, Cornelis
1974SoPh...37...99Z Altcode:
The sunspot models published so far do not reproduce the observed run
of the umbral continuum intensities over the entire spectral range
0.5 < λ < 4 μm. Moreover, in several previous models is the
temperature gradient smaller than both the adiabatic and the radiative
equilibrium gradient.
---------------------------------------------------------
Title: Cooperation in solar Astronomy in Europe
Authors: Zwaan, C.
1974MmSAI..45..929Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: On the Relation Between Moving Magnetic Features and the
Decay Rates of Sunspots
Authors: Zwaan, C.
1974IAUS...56..233Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Magnetische velden en convectie in de zon.
Authors: Spruit, H. C.; Zwaan, C.
1974NTNA...40.....S Altcode:
No abstract at ADS
---------------------------------------------------------
Title: An Efficient Wind Shield for the Protection of Telescopes
Authors: Hammerschlag, R. H.; Zwaan, C.
1973PASP...85..468H Altcode:
A semitransparent wind shield to protect telescopes against wind is
described and some measurements and experiences during a site-testing
campaign are reported. Key words: instrumentation - telescope protection
- seeing
---------------------------------------------------------
Title: The Structure of Sunspots. I: Observational Constraints:
Current Sheet Models
Authors: Gokhale, M. H.; Zwaan, C.
1972SoPh...26...52G Altcode:
As a first step in constructing three-dimensional decaying sunspot
models we select the relevant observational data. From these we
conclude: sunspots, except the smallest, obey a radial and evolutionary
similarity;
---------------------------------------------------------
Title: Empirical NLTE Analyses of Solar Spectral Lines. I: A Method
and Some Applications to Earlier Analyses
Authors: Wijbenga, Jan W.; Zwaan, Cornelis
1972SoPh...23..265W Altcode:
A method is suggested for empirical NLTE analyses of solar spectral
lines. Special depthdependent departure coefficients β(x) are
introduced and the formulas are given for further application. From
test calculations it is shown that the separation of the departure
coefficients for the upper and the lower level from each other
and from the uncertainties in several input parameters (oscillator
strength times abundance, turbulent velocities and damping constant)
is greatly facilitated when spectral lines are analysed on the disk,
around the limb, as well as in the flash spectrum. Therefore it is
necesssary that all accessible line data are used, from accurate line
profiles to equivalent widths or integrated intensities. To reduce
the number of independent variables the analysis should include many
multiplets between a few spectroscopic terms.
---------------------------------------------------------
Title: Magnetic Fields Strengths Derived from Various Lines in the
Umbral Spectrum
Authors: Zwaan, C.; Buurman, J.
1971IAUS...43..220Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Structure of Sunspots
Authors: Zwaan, Cornelis
1968ARA&A...6..135Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Small-Scale Solar Magnetic Fields and 'Invisible Sunspots'
Authors: Zwaan, Cornelis
1967SoPh....1..478Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Note on the normalization of the NBS gf-values and on the
solar abundances of the alkali metals
Authors: Zwaan, C.
1967BAN....19....1Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Sunspot Spectroscopy
Authors: Zwaan, C.
1967sp...conf..229Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Sunspot models : a study of sunspot spectra
Authors: Zwaan, Cornelis
1965RAOU...17.....Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The influence of scattered light on line intensity measurements
Authors: Zwaan, C.
1965IAUS...22..277Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Sunspot models; a study of sunspot spectra
Authors: Zwaan, C.
1965smss.book.....Z Altcode: 1965QB525.Z89......
No abstract at ADS
---------------------------------------------------------
Title: The interpretation of the equivalent widths of atomic and
ionic lines in the spectra of umbrae
Authors: Zwaan, C.
1964susp.conf..169Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The continuous absorption coefficient in the violet and
ultraviolet region
Authors: Zwaan, C.
1962BAN....16..225Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Note on partition functions
Authors: Zwaan, C.
1960AnAp...23..811Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Curves of growth for a large sunspot
Authors: Zwaan, C.
1959BAN....14..288Z Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Note on the Occurence of Technetium in the Solar Atmosphere
Authors: Hubenet, H.; Zwaan, C.; de Jager C.
1954LIACo...5..471H Altcode: 1954MSRSL...1..471H; 1954pna..conf..471H
No abstract at ADS
